This invention relates to a method of patterning a substrate with a micro-printing providing high resolution and to electronic devices, particularly transistors, made with the inventive method.
Demands for low-cost consumer electronics (e.g. large emissive displays, electronic paper, smart cards, and so forth) have precipitated the development of organic materials suitable for use as conducting, semiconducting, and light-emitting materials. Organic materials are attractive for use in electronic devices as they are compatible with plastics and can be easily fabricated to provide low-cost, lightweight, and flexible devices with plastic substrates. Organic materials having a conductivity and carrier mobility permitting their use as the active layer in thin-film semiconductor devices are described in U.S. patent application Ser. No. 08/770,535, filed Dec. 20, 1996, xe2x80x9cMethod of Making An Organic Thin Film Transistor,xe2x80x9d by Zhenan Bao et al. (an inventor herein), also assigned to the present assignee and incorporated herein by reference. An organic device is described in U.S. patent application Ser. No. 09/087,201, filed May 29, 1998, xe2x80x9cThin-Film Transistor Monolithocally Integrated With an Organic Light-Emitting Diode,xe2x80x9d by Zhenan Bao et al. (an inventor herein), which is assigned to the present assignee and incorporated herein by reference. The ""201 application discloses a light emitting diode (LED) having an organic layer as the active layer and a transistor having an organic layer as the semiconductor layer, where the LED and transistor are monolithocally integrated.
Recently, research related to organic materials in electronic devices has expanded to encompass patterning methods that take advantage of the ease with which these materials may be processed. For example, photolithography may be used to pattern organic materials, as described in C. J. Drury et al., APPL. PHYS. LETT. Vol. 73 (1998), at p. 108.
Unconventional, non-photolithographic methods are being explored which provide advantages in terms of cost and flexibility in fabricating semiconductor devices generally. Such techniques include ink jet printing and screen printing which are described in Z. Bao (an inventor herein) et al, xe2x80x9cHigh Performance Plastic Transistors Fabricated by Printing Techniques,xe2x80x9d CHEM. MATER, Vol. 9 (1997), at p. 1299. These non-conventional techniques are suitable for use in a range of fabrication tasks that provide resolution at xcx9c35-100 xcexcm and larger. However, these methods may not be useful for making organic semiconductor devices. For instance, a technique for imprinting patterns on silicon substrates with features having dimensions of 10 to 80 xcexcm using a planar elastomeric stamp has been disclosed, but this technique is not applicable to organic devices. See Kumar and Whitesides, xe2x80x9cFeatures of Gold Having Micrometer to Centimeter Dimensions Can Be Formed Through a Combination of Stamping with an Elastomeric Stamp and an Alkanethiol xe2x80x98Inkxe2x80x99 Followed by Chemical Etching,xe2x80x9d APPL. PHYS. LETT. Vol. 63 (1993), at p. 2002, incorporated herein by reference.
Additionally, the critical dimensions (typically transistor channel lengths) desired for many applications is as low as about 10 xcexcm. Planar stamps are of limited use due to air bubbles that become trapped between the stamp and the substrate. Use of a cylindrical rolling elastomeric stamp is disclosed in Xia, Qin, and Whitesides, xe2x80x9cMicrocontact Printing With a Cylindrical Rolling Stamp: A Practical Step Toward Automatic Manufacturing of Patterns with Submicrometer-Sized Features,xe2x80x9d ADV. MATER. Vol. No. 12 (1996), at p. 1015, incorporated herein by reference. Microcontact printing to print SAMS patterns is further described in D. Qin, Y. Xia, J. A. Rogers (an inventor herein), et al., xe2x80x9cMicrofabrication, Microstructures and Microsystems,xe2x80x9d TOPICS IN CURRENT CHEMISTRY (1998), at pp. 1-20, which is incorporated herein by reference. These methods do not provide structures that are immediately useful in making organic semiconductor devices. The method described in the Advanced Materials publication does not allow for continuous printing and provides only a single impression from lip the patterned stamp.
A fabrication strategy has been recently developed to obtain high resolution devices. This strategy involves a combined micromolding/screen-printing strategy, i.e., this strategy combines an emerging high-resolution technique (micromolding in capillaries) for defining critical features and an established low-resolution method (screen printing) for patterning other elements of the devices. See, e.g., J. A. Rogers, Z. Bao (inventors herein) and V. R. Raju, APPL. PHYS. LETT. Vol. 71 (1998), at p. 2716. With this strategy, organic transistors are produced with channel lengths approximately fifty times smaller (xcx9c2 microns) than those previously achieved with other non-photolithographic schemes, providing a resolution exceeding the requirements for most applications. However, the micromolding can be relatively slow (xcx9c30 minutes may be needed to pattern a substrate). Also, it is not compatible with reel-to-reel processing or readily applied to construct dense, non-interconnected circuitry.
As may be appreciated, those concerned with technologies involving electronics devices or systems continue to search for new methods of patterning substrates to enable faster and more flexible processing of the materials to provide less-expensive and improved devices. In particular, it would be advantageous to provide improved high-resolution patterning techniques that are useful in making organic devices that are low in cost and applicable to fabricate dense, non-interconnected circuitry. This invention provides these advantages.
Summarily described, the invention embraces a method for patterning a layer of material on a substrate at high resolution that is useful in making electronic devices, including organic electronic devices. The method comprises a step of providing a rotatable stamp having relief geometries on its surface that advantageously may comprise a cylindrical stamp mounted on an axle. Ink is applied to the surface of the stamp, preferably with a cylindrical inking pad to provide substantially continuous inking of the stamp. The cylindrical stamp is rolled against a substrate, which preferably is provided in the form of a sheet of material, so that an inked pattern is impressed on the substrate. The substrate is then passed to an apparatus for etching material from or depositing material on the substrate. The inked pattern on the substrate guides the etching or deposition of material to form a patterned layer. The substrate optionally may be sent to a mechanism for removing the inked pattern from the substrate, which may comprise exposing the substrate to UV light, heat, or a wet chemical means.